1. Field of the Invention
The present invention relates to an image forming apparatus, and more particularly to an apparatus which controls the transport speed of a sheet.
2. Description of the Related Art
FIG. 7 shows the construction of a conventional image forming apparatus. Referring to FIG. 7, numeral 1 denotes an image forming section for forming an image through an electrophotographic process. The image forming section 1 comprises a photoconductive drum 10, a laser scanner 9 for illuminating light to the surface of the photoconductive drum 10 to form an electrostatic latent image on it, and a developing station 11 for developing the latent image formed on the photoconductive drum 10.
Also, numeral 2 denotes a cassette for accommodating sheets of recording paper 3, and numeral 6 denotes a paper supply transport section for feeding the sheets of recording paper 3 to the image forming section 1 one by one. The paper supply transport section 6 comprises a reversing guide 7 for introducing the recording paper 3 to the image forming section 1 in a reversed state, register rollers 8 for correcting a skew of the recording paper 3. Numeral 14 denotes a paper ejection transport section for transporting the recording paper 3, on which an image has been formed in the image forming section 1, to a paper ejection tray 17 that serves as a paper ejecting section. The paper ejection transport section 14 comprises a reversing guide 15 and a paper ejection roller 16 for introducing the recording paper 3 to the paper ejection tray 17.
When an image is formed in the image forming apparatus thus constructed, one of the sheets of recording paper 3 accommodated in the cassette 2 is first picked up by a pickup roller 4 and then separated by a separator 5 for feeding to the paper supply transport section 6. Then, the recording paper 3 passes the reversing guide 7 and is introduced to the image forming section 1 after a skew of the recording paper 3 has been corrected by the register rollers 8.
Subsequently, when the recording paper 3 passes a top sensor 18 disposed immediately upstream of the image forming section 1, the laser scanner 9 for the image forming section 1 starts operation in synchronization. The laser scanner 9 irradiates a laser beam onto the photoconductive drum 10 to form an electrostatic latent image thereon. Then, the electrostatic latent image is developed into a visible image and transferred to the recording paper 3 in a transferring station 12.
After the transfer of the visible image, the recording paper 3 is guided to a fusing station 13 by a guide 19. The visible image is fused and fixed in the fusing station 13 under heating and pressure, whereby the image formation is completed. Then, the recording paper 3 is advanced to the paper ejection transport section 14 and ejected onto the paper ejection tray 17 by the paper ejection roller 16 after passing the reversing guide 15.
In such a conventional image forming apparatus, efforts have been made to transport the recording paper 3 at a sheet transport speed in the paper supply transport section 6 and the paper ejection transport section 14 about twice that in the image forming section 1, i.e., a process speed, for the purpose of reducing a fast copying time.
However, efforts at increasing the sheet transport speed have caused problems. When outputting 20 copies of the same original document successively, for example, an increase of the sheet transport speed in the paper supply transport section 6 and the paper ejection transport section 14 generates increased noise due to an increase in frictionally sliding sounds between the recording paper 3 and the reversing guides 7, 15, to collision sounds generated due to backlashes in engagement between gears at the time of start and stop, to collision sounds from clutches for transmitting torques to transport rollers, to sounds occurring when the recording paper 3 strikes against the register rollers 8 when a skew of the recording paper 3 is compensated for, and to sounds occurring when the recording paper 3 strikes against the reversing guide 7 when the recording paper 3 is looped. In particular, when the reversing guides 7, 15 are curved as shown in FIG. 7, the frictionally sliding sounds between the recording paper 3 and the reversing guides 7, 15 become more noticeable.
Further, a larger current must be supplied to each motor, etc., for achieving increased speed. This necessarily increases the amount of heat generated by the motor and raises motor temperature. Such increase of motor temperature not only deteriorates reliability of the motor itself, but also raise a temperature in the apparatus body. This temperature rise in the apparatus body causes an adverse effect upon the image forming section.
A fan or the like may be used to cool the image forming section for suppressing the adverse effect due to such a temperature rise. This case however would increase the costs. In the case of slowing down the sheet transport speed for the same purpose, overall productivity (time required for completion of a print) would be reduced.
In view of the state of the art set forth above, it is an object of the present invention to provide an image forming apparatus capable of suppressing noise without reducing productivity.
To achieve the above object, according to one aspect of the present invention, there is provided an image forming apparatus comprising an image forming section, a paper supply transport section for transporting a sheet to the image forming section, and a paper ejection transport section for transporting the sheet, on which an image has been formed in the image forming section, to a paper ejection section, the apparatus further comprising a sheet transporting unit provided in each of the paper supply transport section and the paper ejection transport section; and a speed control unit for controlling sheet transport speeds of each of the sheet transporting units, the speed control unit controlling the sheet transport speed of at least one of the sheet transporting units provided in the paper supply transport section and the paper ejection transport section to be higher than the sheet transporting speed in the image forming section when an image is formed on a predetermined sheet.
Also, according to the present invention, an image forming apparatus comprises an image forming section, a paper supply transport section for transporting a sheet to the image forming section, a sheet transporting unit provided in the paper supply transport section, and a speed control unit for controlling a sheet transport speed of the sheet transporting unit, the speed control unit controlling the sheet transport speed of the sheet transporting unit provided in the paper supply transport section to be higher than a sheet transporting speed in the image forming section during image formation.
Further, according to the present invention, the predetermined sheet is the last or first one when images are successively formed on a plurality of sheets.
Still further, according to the present invention, each sheet transporting unit is driven by a stepping motor, and the speed control unit controls the sheet transport speed of each sheet transporting unit by controlling the respective stepping motor.
Still further, according to the present invention, the paper supply transport section or the paper ejection transport section includes a curved sheet transport path.
Still further, according to the present invention, the image forming apparatus further comprises a re-transport section for transporting a sheet, on one side of which an image has been formed, back to the image forming section for forming an image on the other side of the sheet having the image formed on one side thereof; and a sheet transporting unit provided in the re-transport section, the speed control unit controlling the sheet transport speed of the sheet transporting unit provided in at least one of the re-transport section, the paper supply transport section and the paper ejection transport section to be higher than the sheet transport speed in the image forming section during image formation on the other side of the last sheet.
Still further, according to the present invention, a paper ejecting unit for ejecting a sheet to the paper ejection section is provided in the paper ejection transport section, and the speed control unit controls the paper ejecting unit to eject the sheet at a predetermined sheet ejection speed regardless of the sheet transport speed in the paper ejection transport section.
Thus, according to the present invention, the sheet transporting units are provided in the paper supply transport section for transporting a sheet to the image forming section, and in the paper ejection transport section for transporting the sheet, on which an image has been formed in the image forming section, to the paper ejection section. When an image is formed on a predetermined sheet, the speed control unit controls the sheet transport speed of at lease one of the sheet transporting units provided in the paper supply transport section and the paper ejection transport section to be higher than that in the image forming section.
Further objects, features and advantages of the present invention will become apparent from the following description of the preferred embodiments with reference to the attached drawings.